1. Field of the Invention
The present invention relates to a lithium secondary battery, and more particularly, to a jelly-roll type battery unit configured such that an electrode tab is incorporated into an electrode current collector and having an improved structure in which electrode plates are disposed, a winding method of the battery unit and a lithium secondary battery employing the battery.
2. Description of the Related Art
Unlike a primary battery incapable of charging, secondary batteries are generally capable of charging and discharging. Lithium batteries are in a widespread use in advanced electronic devices such as cellular phones, notebook-type computers, camcorders and the like, and are classified into nickel-cadmium (Ni—Cd) batteries, nickel-hydride (Ni-MH) batteries and lithium secondary batteries. Specifically, the lithium secondary batteries are rapidly developing in view of their high operating voltage of 3.6 V or higher, which is approximately 3 times that of the nickel-cadmium (Ni—Cd) batteries or nickel-hydride (Ni-MH) batteries, and their excellent energy density per unit weight. In recent years, lithium secondary batteries have attracted attention since they have a high energy density per unit weight and an operating voltage of 3.6 V or greater, which is three times higher than nickel-cadmium (Ni—Cd) batteries, nickel-metal hydride (Ni-MH) batteries and nickel-hydrogen batteries.
Lithium secondary batteries use lithium oxide as a positive electrode active material and a carbon material as a negative electrode active material. The lithium secondary batteries may be classified into liquid electrolyte batteries and solid electrolyte batteries according to the electrolyte used. In general, batteries using a liquid electrolyte are referred to as lithium-ion batteries and batteries using a polymeric electrolyte are referred to as lithium polymer batteries. The lithium secondary batteries may be manufactured in various shapes, typically in cylindrical, rectangular or pouchy shapes.
FIG. 1 shows a winding start portion of a battery 10 disclosed in U.S. Pat. No. 5,508,122 and FIG. 2 shows a winding completion portion of the battery 10 shown in FIG. 1.
Referring to the drawings, the battery 10 includes a positive electrode plate 1, a negative electrode plate 2 and a separator 3.
The positive electrode plate 1 includes a positive electrode current collector 11 and positive electrode slurry coated on both surfaces of the positive electrode current collector 11. A positive electrode tab 12 is attached to a portion of the positive electrode plate 1 where the positive electrode slurry is not coated. The positive electrode current collector 11 and the positive electrode tab 12 are both made of aluminum.
The negative electrode plate 2 includes a negative electrode current collector 21 and negative electrode slurry coated on both surfaces of the negative electrode current collector 21. A negative electrode tab 22 is attached to a portion of the negative electrode plate 2 where the negative electrode slurry is not coated. The negative electrode current collector 21 is made of a copper foil and the negative electrode tab 22 is made of nickel.
In the battery 10 having the above-described configuration, the separator 3, the positive electrode plate 1 and the negative electrode plate 2 are wound around a winding spool 4 in that order. The negative electrode plate 2 is wound onto the electrode unit that is greater than or equal to 15 mm after the beginning of the positive electrode plate 1. Thus, the surface of the positive electrode tab 12 faces the positive electrode current collector 11 with the separator 3 disposed therebetween.
As to the winding completion portion of the battery 10, the surface of the negative electrode tab 22 faces the separator 3, an insulating tape 5 is attached to the end portion of the positive electrode slurry coated portion and winding of the negative electrode tab 22 is then performed. Thus, a vortex of electrodes may be prevented, and the positive electrode plate 1 may not be positioned at a portion facing the negative electrode tab 22.
However, the conventional battery 10 has the following problems.
The negative electrode tab 22 made of a nickel plate is attached to an area of the negative electrode plate 2 made of a copper foil where a negative electrode slurry is not coated by ultrasonic welding. Here, since the negative electrode plate 2 has the negative electrode tab 22 made of a different metal from the same, it is prone to deformation during charge and discharge.
In particular, in the case of using a strip-shaped electrode plate, adhesiveness between each electrode plate and a separator may be lowered due to foreign matter during winding, producing a non-charged area and deforming a jelly-roll type battery unit, thus affecting the thickness of the battery 10.
Since the negative electrode tab 22, which is separately provided, is welded on the negative electrode plate 2, the material cost increases due to consumption of the negative electrode tab 22. Also, use of different metals increases internal resistance.